1. Field of the Invention
The present invention relates to a high frequency tuning amplifier for buffer and particularly to a high frequency tuning amplifier for buffer which is used in common for a first voltage controlled oscillator to generate the high band frequency and a second voltage controlled oscillator to generate the low band frequency to be selectively tuned to amplify the high band frequency and low band frequency.
2. Description of the Related Art
In regard to the mobile communication systems used in the world, DCS (Digital Cellular System) is introduced in Britain, Germany, Italy, France and a part of Asian Countries, while GSM (Global System for Mobile Communications), which is the standard system for European countries for digital mobile telephone system signed in 1982, is employed in European countries, USA, Africa and a part of Asian countries.
In this case, DCS is called a mobile communication system in which a base station frequency is determined as 1805 MHz to 1880 MHz, a mobile station frequency as 1710 MHz to 1785 MHz, an oscillation frequency of a voltage controlled oscillator (VCO) of a portable telephone as 1700 MHz, total of 374 channels are used and GMSK (Gaussian Minimum Shift Keying) system is used as the modulation system. Moreover, GSM is called a mobile communication system in which a base station frequency is determined as 925 MHz to 960 MHz, a mobile station frequency as 880 MHz to 915 MHz, an oscillation frequency of a voltage controlled oscillator (VCO) of a portable telephone as 900 MHz, total of 124 channels are used and GMSK is also used as the modulation system.
Since these two mobile communication systems DCS and GSM are intrinsically mobile communication systems based on different systems, when a user hopes to become a subscriber of these two mobile communication systems, a user is requested to get two types of portable telephones to make mobile communication by the DCS and GSM systems.
However, since DCS and GSM are only different in the available frequency assigned but introduce GMSK system as the modulation system used, a portable telephone which can be used for both DCS and GSM systems is already proposed. In the case of this related art, a first voltage controlled oscillator which generates the 1700 MHz frequency band and a second voltage controlled oscillator which generates the 900 MHz frequency band are arranged within the portable telephone and thereby the first voltage controlled oscillator is used for the mobile communication by the DCS system, while the second voltage controlled oscillator is used through the switching operation for the mobile communication by the GSM system.
Here, FIG. 6 is a block diagram illustrating a structure of layout of two voltage controlled oscillators in the portable telephone of the related art which may be used in both DCS and GSM systems.
As illustrated in FIG. 6, a portable telephone which may be used both in DCS and GSM systems is composed of a first voltage controlled oscillator (first VCO) 61 to generate the frequency of 1700 MHz band (high band frequency), a second voltage controlled oscillator (second VCO) 62 to generate the frequency of 900 MHz band (low band frequency), a first high frequency tuned amplifier 63 for buffer to selectively amplify the high band frequency signal, a second high frequency tuned amplifier 64 for buffer to selectively amplify the low band frequency signal, a high band frequency signal output terminal 65, a low band frequency signal output terminal 66, a power source 67 and a 1-circuit/2-contact switch 68.
The first voltage controlled oscillator 61 is connected at its output terminal to the input terminal of the first high frequency tuned amplifier 63 for buffer, while the power source terminal is connected to one fixed contact of the 1-circuit/2-contact switch 68. The second voltage controlled oscillator 62 is connected at its output terminal to the input terminal of the second high frequency tuned amplifier 64 for buffer, while the power source terminal is connected to the other fixed terminal of the 1-circuit/2-contact switch 68. The first high frequency tuned amplifier 63 for buffer is connected at its output terminal to the high band frequency signal output terminal 65. The second high frequency tuned amplifier 64 for buffer is connected at its output terminal to the low band frequency signal output terminal 66. The power source 67 is connected at its positive output terminal to the movable contact of the 1-circuit/2-contact switch 68 and is also connected at its negative output terminal to the grounding point.
In the above structure, when the portable telephone is used for mobile communication by the DCS system, the movable contact of the 1-circuit/2-contact switch 68 is connected by the switching operation to one fixed contact. In this case, the power source 67 supplies a drive power to the first voltage controlled oscillator 61 to operate the first voltage controlled oscillator 61 in order to output the high band frequency (1700 MHz band) therefrom. This high band frequency signal is selectively amplified by the first high frequency tuning amplifier for buffer and is then supplied to the available circuits from the high band frequency signal output terminal 65. Meanwhile, when the portable telephone is used for mobile communication by the GSM system, the movable contact of the 1-circuit/2-contact switch 68 is connected by the switching operation to the other fixed contact. In this case, the power source 67 supplies a drive power to the second voltage controlled oscillator 62 to operate the second voltage controlled oscillator 62 in order to output the low band frequency (900 MHz band) signal from the second voltage controlled oscillator 62. This low band frequency signal is selectively amplified by the second high frequency tuned amplifier 64 for buffer and is then supplied to the available circuits from the low band frequency signal output terminal 66.
In this case, the reason why the first high frequency tuned amplifier 63 for buffer is connected in the output side of the first voltage controlled oscillator 61 and the second high frequency tuned amplifier 64 for buffer in the output side of the second voltage controlled oscillator 62 is that it is difficult to obtain the high frequency tuned amplifier for common use having almost equal gain characteristic and passing band characteristic in both high band frequency and low band frequency.
The existing portable telephone which may be used in common for both DCS and GSM systems has a problem that since the first high frequency tuned amplifier 63 for buffer is connected in the output side of the first voltage controlled oscillator 61 and the second high frequency tuned amplifier 64 for buffer is connected in the output side of the second voltage controlled oscillator 62, two kinds of high frequency tuned amplifiers of the first high frequency tuned amplifier 63 for buffer and second high frequency tuned amplifier 64 for buffer are required and it is required to reserve the space for arrangement of two kinds of high frequency tuned amplifiers within the portable telephone and therefore it is difficult to reduce the size of portable telephone and moreover manufacturing cost becomes high.